If Mode is set to TwoWay, the NumericUpDown is enabled/disabled as the check box is checked/unchecked. A text box with Mode=OneWay works as expected.
<dataForm:DataField Label="Override">
<StackPanel Orientation="Horizontal">
<CheckBox x:Name="_enableCheckBox"/>
<TextBox IsEnabled="{Binding IsChecked, ElementName=_enableCheckBox, Mode=OneWay}" Width="75" />
<toolkitInput:NumericUpDown IsEnabled="{Binding IsChecked, ElementName=_enableCheckBox, Mode=OneWay}" Width="75" />
</StackPanel>
</dataForm:DataField>